Many brackets for connecting suspension links, stabilizer bars or harnesses and the like are attached to a sub-frame of a suspension, and connection by fastening using fastening parts such as bolts and nuts is made. After the nuts are welded to the sub-frame in advance at each connection part, only bolts are installed with a wrench, and use of such a method achieves excellent work efficiency. In the case where the nut is welded to the sub-frame, a so-called projection nut having a projection on the nut seating surface is employed, and measures of attaching the nut to a bracket seating surface of the sub-frame by projection welding are often used.
In general, a conventional apparatus used for welding the nut to a counterpart component with projection includes a feeder for feeding a nut, a component feeding part, an upper electrode, and a lower electrode (refer to Patent Literature 1).